1. Field of the Invention
The invention relates to a battery pack which has both a primary battery and a secondary battery such as a lithium ion battery or the like and which enables the secondary battery to be charged by the primary battery.
2. Description of the Related Arts
Secondary batteries are widely used in portable electronic apparatuses. Since a lithium ion battery is weak against an overcharge or an overdischarge, in general, it has a construction of a battery pack in which a battery cell and a protecting circuit are integrated. The protecting circuit has three functions of protection against the overcharge, the overdischarge, and an overcurrent. Those protecting functions will be simply explained hereinbelow.
The overcharge protecting function will now be described. When the lithium ion battery is charged, a battery voltage continues to rise even after exceeding a full charge state. When the battery enters such an overcharge state, a possibility that the battery enters a dangerous state occurs. Therefore, it is necessary to charge the battery at a constant current and a constant voltage and by a charge control voltage that is equal to or less than a rated value (for example, 4.2 V) of the battery. However, there is a risk of occurrence of the overcharge due to a failure of a charger or use of a charger for a different kind of battery. When the battery is overcharged and the battery voltage is equal to a certain value or more, the protecting circuit turns off a charge control FET, thereby shutting off a charge current. Such a function is the overcharge protecting function.
The overdischarge protecting function will now be described. When the battery is discharged to a rated discharge terminating voltage or less and enters the overdischarge state where the battery voltage is equal to, for example, 2 to 1.5 V or less, there is a case where the battery fails. When the battery is discharged and the battery voltage is equal to a certain voltage value or less, the protecting circuit turns off a discharge control FET, thereby shutting off a discharge current. Such a function is the overdischarge protecting function.
The overcurrent protecting function will now be described. When (+) and (−) terminals of the battery are short-circuited, a large current flows, so that there is a risk of abnormal heat generation. When a discharge current of a certain value or more flows, the protecting circuit turns off the discharge control FET, thereby shutting off the discharge current. Such a function is the overcurrent protecting function.
A conventional secondary battery pack is constructed only by a secondary battery and it is necessary to charge it by using a dedicated charger from a power source for household use at home or the like. Therefore, when a residual capacity of the battery is extinguished at a place where the user has gone to, there is such inconvenience that portable electronic apparatuses such as cellular phone, PDA (Personal Digital Assistants), digital camera, camcoder (a general abbreviated name of “camera and recorder”, video camera+recorder), and the like cannot be used.
As one of methods of solving such a problem, in cellular phones, a charging apparatus comprising only a primary battery which can be connected to a main body has been put into practical use. However, there is such a problem that the user has to separately carry a charging pack. To solve such a problem, a battery pack in which both a primary battery and a secondary battery are integrated is used and when a voltage of the secondary battery drops, the secondary battery is charged by the primary battery has been disclosed in JP-A-2000-324703.
According to the battery pack disclosed in JP-A-2000-324703, a comparator for comparing a voltage of the primary battery with that of the secondary battery is provided and when the voltage of the primary battery is larger than that of the secondary battery, the secondary battery is charged by the voltage of the primary battery through a regulator.
According to the battery pack disclosed in JP-A-2000-324703, although there is a disclosure that the battery is charged by an external charger, the battery pack does not have a function for automatically switching the primary battery as a charging power source and the external charger and a voltage from the external charger is not controlled. Further, when the battery is charged by the primary battery, the voltage of the primary battery is controlled by the regulator. If the voltage is controlled by the regulator, efficiency is low and a large electric power is wastefully consumed. Further, since the regulator is not small, a circuit cannot be miniaturized by the method of controlling by the regulator,